A variable valve train that uses a hydraulic actuator to vary the valve opening characteristic of intake and exhaust valves of an internal combustion engine is widely used. For example, there is a known variable valve timing mechanism, which includes a hydraulic actuator for rotating a camshaft relative to a timing gear and varies the valve open/close timing while maintaining a fixed operating angle (valve opening period).
A common method of controlling the above variable valve train is to exercise feedback control over a duty value of a control signal for a hydraulic control valve, which controls the oil pressure acting on the hydraulic actuator, in accordance with a deviation between a target valve opening characteristic and an actual valve opening characteristic.
However, when, for instance, oil is low in temperature and high in viscosity (when the engine is cold) or when the oil is deteriorated, the oil flow resistance in a hydraulic circuit and the friction resistance of each slide member increase. This may readily exert an adverse effect on the motions of the hydraulic actuator and hydraulic control valve. As a result, the responsiveness of the variable valve train may decrease. Thus, the following conventional technologies are proposed to solve the above problem.
When oil temperature is lower than a normal temperature region, a technology disclosed in JPA-2006-244230 ensures that the frequency of a pulse width modulated signal, which is a control signal for a hydraulic control valve, is lower than a normal frequency.
When there is a great deviation between the target valve opening characteristic and actual valve opening characteristic at a low oil temperature, a technology disclosed in JP-A-2003-254017 does not exercise feedback control over a control signal duty value for a hydraulic control valve, but exercises inching control so that an operation for keeping the duty value at a forced drive duty value (e.g., 100% or 0%) for a predetermined period of time is repeatedly performed at predetermined time intervals.
[Patent Document 1] JP-A-2006-244230
[Patent Document 2] JP-A-2003-254017
[Patent Document 3] JP-A-2006-170011
[Patent Document 4] JP-A-10-227235
[Patent Document 5] JP-A-11-236831
[Patent Document 6] JP-A-11-2142